Portable electronic devices such as portable two-way radio devices are typically designed with size being an important consideration. It is desirable to keep the size of the device as small as is reasonably possible, given other considerations. This necessarily means the electrical and electronic components are arranged to be in close proximity to each other inside the device. While this is a conventional design approach for most applications, it poses some issues for applications where a device is to be used safely in an atmosphere including volatile components. Such potentially hazardous conditions can be found, for example, in chemical processing operations, mining, petroleum refineries, and so on. In the event such conditions arise, people in such areas may need to communicate. Accordingly, communication devices for use in such conditions need to be designed so as to avoid ignition of volatile components that may be dispersed in the atmosphere around the device.
The term “intrinsic safety” refers to the design of devices such that they are intrinsically incapable of causing ignition of volatile atmospheric components (e.g. gases, fumes, dust). To accomplish this there are several considerations that must be addressed, and among them are energy storage and the ability of a component to reach a temperature that can cause ignition. Energy storage refers to, for example, the storage of charge in capacitors and the storage of flux in inductive components. In considering a given design it is assumed that storage components can experience faults (i.e. sudden short or open circuit conditions) which cause near-instant release of the energy stored in the component. Accordingly, the ability of a design to store energy must be such that a sudden release of energy cannot cause a sufficiently energetic event to cause ignition. Similarly, components are examined to determine their thermal response to fault conditions, and whether the component can achieve an ignition temperature.
There are various ways for dealing with potential fault conditions in the design on intrinsically safe products. One way is to use redundant limiting circuits that prevents voltage and/or current levels above a threshold that could potentially result in excessive energy storage. Another way is to use oversized components for heat dissipation to ensure that even under fault conditions a particular component cannot achieve a sufficiently high temperature. However such circuits add to the already complex circuitry of the device, and can cause signal degradation.
Another means for dealing with potential fault conditions and thermal considerations is to encapsulate circuit components to reduce the chance of fault conditions and to exclude gasses or other volatile airborne components from being in contact with circuit components. Conventional encapsulation techniques involve spraying an encapsulant material onto circuit boards, or dipping circuit boards in encapsulant material until a sufficiently thick layer of encapsulant can be cured on the circuit board covering circuit components. While the result of this method of encapsulation is effective, it consumes a considerable amount of space in a multi-circuit board architecture due to each of the circuit boards being separately encapsulated. Furthermore, additional protection on the connections between circuit boards (i.e. connectors, flex, pins, etc) is required on both side of the circuit boards. it is not desirable from a manufacturing point of view due to variances in the process and amount of handling that is required.
Accordingly, there is a need for a method and apparatus for encapsulating circuitry for intrinsically safe applications that provides a space saving solution which avoids the need for additional protection circuits on the connections between circuit boards, and which provides the necessary robustness on the surface of the encapsulant material.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.